Fair Value Measurements (Details) (USD $)	Dec. 31, 2011	Sep. 30, 2011
Fair Value Abstract
Cash Collateral Right To Reclaim Cash	$ 22,100,000	$ 28,800,000
Debt Instrument Fair Value	2,572,094,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	11,400,000	12,400,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	10,700,000	16,400,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	33,768,000	15,262,000
Total financial instruments	33,768,000	15,262,000
Hedged portion of gas stored underground	77,551,000	47,940,000
Available For Sale Securities Equity Securities	38,008,000	36,444,000
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	38,008,000	36,444,000
Total assets	149,327,000	99,646,000
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	51,117,000	22,091,000
Total liabilities	51,117,000	22,091,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	472,000	1,841,000
Nonregulated Segement Derivative Assets	256,098,000	97,396,000
Total financial instruments	256,570,000	99,237,000
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	18,346,000	14,366,000
Available-for-sale securities money market funds	1,151,000	1,823,000
Fair value	19,497,000	16,189,000
Total assets	276,067,000	115,426,000
Natural gas distribution segment	86,301,000	81,118,000
Nonregulated Segement Derivative Liabilities	254,012,000	115,617,000
Total liabilities	340,313,000	196,735,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	0	0
Total financial instruments	0	0
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	0	0
Total assets	0	0
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	0	0
Total liabilities	0	0
Netting And Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	(272,938,000)	(95,156,000)
Total financial instruments	(272,938,000)	(95,156,000)
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	0	0
Total assets	(272,938,000)	(95,156,000)
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	(295,022,000)	(123,943,000)
Total liabilities	$ (295,022,000)	$ (123,943,000)